This invention relates to actuators and actuator assemblies.
The invention is more particularly, but not exclusively, concerned with linear actuators.
Linear actuators have a rod that is movable into and out of a housing, under control of an electrical or other input signal, so as to effect linear displacement of a member coupled to the end of the rod. In some applications, such as in aircraft, where a high degree of reliability and safety is required, two actuators are commonly coupled together to form an assembly which is capable of effecting a similar displacement even when one of the actuators fails or malfunctions. The two actuators may be joined together back-to-back at their housings so that the respective rods are displaced in opposite directions by the input signals. In this way, each actuator contributes half of the total displacement effected by the assembly.
In order to ensure that the assembly is still capable of functioning satisfactorily if one of the actuators fails, it is necessary that the rod of the failed actuator locks in position relative to its housing. If this did not happen, any displacement caused by the correctly functioning actuator might merely be accommodated by displacement of the rod in the housing of the failed actuator. Present actuators are therefore constructed so that the rod locks in position immediately on failure, that is, in the position at which the actuator fails.
Although such actuators do enable the assembly to provide some degree of control if one actuator has failed, they suffer from a significant disadvantage. If one actuator in an assembly fails when the assembly is providing maximum displacement, such as, with both rods fully out of their housings, one rod will be locked in this position, so that, even if the other rod is fully retracted within its housing it will only just return the member coupled to the actuator to a central position. Since the most critical control is generally required about the central position of the member, it will be appreciated that the actuator assembly, in this failed state, will not provide the desired degree of control.